Inspection of manufactured products helps to reduce the number of defective products introduced into the marketplace. The effectiveness of certain types of inspections, for example inspections of printed circuit boards or printed wiring boards (collectively "PWBs"), is often dependent upon the illumination systems and processes used to provide inspection lighting of the manufactured product during inspection.
This is especially true for automated inspection of PWBs using line scan cameras--the ability to identify, classify, and properly detect defects in the inspected PWBs is greatly affected by the lighting available on the subject PWB. Proper illumination allows for enhanced visibility of PWB manufacturing defects, including raw paste analysis (coverage and volume, missing/mis-aligned/damaged components, bent/missing/lifted leads, bridges and near bridges, alignment gauging, insufficient and excess solder, solder dewets, cold and fractured solder joints, etc.).
Several different illumination methodologies have been used for PWB inspection as well as other inspection tasks. These illumination techniques include highly diffuse ("cloudy day") illumination, direct illumination, re-directed illumination using fiber optics to distribute the source over a line, projected images of lines, ring lights, coaxial illumination, flood lights, as well as all of the above with some form of additional diffuser. Each of these methods proves inadequate when used for automated inspection of PWBs using a line scan camera for at least one or more of the following reasons: (1) inadequate overall light level; (2) excessive localized blooming or glinting (i.e., saturation of camera imaging elements due to excessive light); (3) insufficient coverage of light to fill shadows; and (4) image shiny, near vertical objects, such as leads.
For example, fixed angle single light sources cannot provide lighting to shadowed areas (such as shorter components surrounded by taller components or areas under vertical leads perpendicular to the scan direction). In order to get the light intensity required for inspection, these fixed angle light sources can also cause "blooming" or very bright spots in the camera (due to the angle of reflection) that prevent inspection of that area because nothing could be discerned from the blooming area. In addition, because solder fillets are "mound-shaped", regardless of the light angle variation, some reflection in a specular fashion back to the camera will occur. To a line scan camera, the shadow areas often look black while the shiny or blooming areas look white and accurate detection of defects in these areas is difficult or impossible.